Conventional implantable stimulation devices for cardiac therapy often sense information associated with cardiac performance. A care provider may use such information to adjust cardiac stimulation therapy or to recommend other therapy while some implantable stimulation devices can use such information to automatically adjust therapy (e.g., according to an adjustment algorithm). In either instance, the amount of information available to the care provider or the adjustment algorithm may be limited by storage capabilities of the implantable stimulation device. For example, if a device measures and stores heart rate every minute, then after one week the device would have stored over 10,000 heart rate values. For a consultation schedule of once every three months, the device would have stored about 130,000 heart rate values. For heart rate values of 8 bits each (e.g., 1 byte), the storage requirements would be about 130,000 bytes. While some conventional implantable stimulation devices may have storage capabilities in excess of 100,000 bytes, choices still need to be made as to how to best use the limited storage capabilities. Thus, some devices rely on data reduction techniques to reduce storage needs.
Data reduction techniques include averaging, binning, modeling, etc. In the above example, a running average of 1 hour for heart rate values will reduce storage needs by a factor of 60. Binning, which is usually associated with histograms, can also reduce storage needs. For example, if a patient experiences a heart rate of 60 bpm one thousand times over a three month period, binning would store a representation of 1,000 in a 60 bpm bin as opposed to storing all 1,000 heart rate values. In this example, however, time may be lost in that the binning does not register a time for each bin count.
In general, choices need to be made that balance requirements for diagnostic information and storage capabilities. As described below, various exemplary data reduction algorithms aim to retain the diagnostic value of information while optionally reducing storage requirements.